kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 71 insertions, 0 deletions

diff --git a/Documentation/fb/efifb.rst b/Documentation/fb/efifb.rst
new file mode 100644
index 000000000..3d4aab406
--- /dev/null
+++ b/Documentation/fb/efifb.rst
@@ -0,0 +1,71 @@
+===================================
+efifb - Generic EFI platform driver
+===================================
+
+This is a generic EFI platform driver for systems with UEFI firmware. The
+system must be booted via the EFI stub for this to be usable. efifb supports
+both firmware with Graphics Output Protocol (GOP) displays as well as older
+systems with only Universal Graphics Adapter (UGA) displays.
+
+Supported Hardware
+==================
+
+- iMac 17"/20"
+- Macbook
+- Macbook Pro 15"/17"
+- MacMini
+- ARM/ARM64/X86 systems with UEFI firmware
+
+How to use it?
+==============
+
+For UGA displays, efifb does not have any kind of autodetection of your
+machine.
+
+You have to add the following kernel parameters in your elilo.conf::
+
+	Macbook :
+		video=efifb:macbook
+	MacMini :
+		video=efifb:mini
+	Macbook Pro 15", iMac 17" :
+		video=efifb:i17
+	Macbook Pro 17", iMac 20" :
+		video=efifb:i20
+
+For GOP displays, efifb can autodetect the display's resolution and framebuffer
+address, so these should work out of the box without any special parameters.
+
+Accepted options:
+
+======= ===========================================================
+nowc	Don't map the framebuffer write combined. This can be used
+	to workaround side-effects and slowdowns on other CPU cores
+	when large amounts of console data are written.
+======= ===========================================================
+
+Options for GOP displays:
+
+mode=n
+        The EFI stub will set the mode of the display to mode number n if
+        possible.
+
+<xres>x<yres>[-(rgb|bgr|<bpp>)]
+        The EFI stub will search for a display mode that matches the specified
+        horizontal and vertical resolution, and optionally bit depth, and set
+        the mode of the display to it if one is found. The bit depth can either
+        "rgb" or "bgr" to match specifically those pixel formats, or a number
+        for a mode with matching bits per pixel.
+
+auto
+        The EFI stub will choose the mode with the highest resolution (product
+        of horizontal and vertical resolution). If there are multiple modes
+        with the highest resolution, it will choose one with the highest color
+        depth.
+
+list
+        The EFI stub will list out all the display modes that are available. A
+        specific mode can then be chosen using one of the above options for the
+        next boot.
+
+Edgar Hucek <gimli@dark-green.com>